Method of bonding a semiconductor to a package with a low and high viscosity bonding agent

ABSTRACT

A semiconductor device comprising a semiconductor element, a support member for supporting the semiconductor element on the support member. The bonding layer comprises element and a bonding layer for bonding the semiconductor a hard layer of a high-viscosity bonding agent which has a high viscosity before curing and which has an irregular free surface defining projections and cavities, and a soft layer of a low-viscosity bonding agent which has a low viscosity before curing and disposed within the cavities in the irregular free surface of the hard layer to fill them. The hard layer defining a portion of the bonding surfaces and the soft layer defining the remaining portion of the bonding surface provide a continuous hybrid bonding surface for the bonding layer. A process for bonding a semiconductor element to a support member is also disclosed.

BACKGROUND OF THE INVENTION

This invention relates to a semiconductor device in which asemiconductor element is bonded to a support structure and a process forbonding a semiconductor element to a support structure.

The die bonding for use in bonding a die (semiconductor element) to asemiconductor element support structure such as a package or a substratetypically includes a gold-silicon eutectic bonding and a solder bonding.In recently developed devices such as color image sensors including acolor filter which are less heat resistant applying the low-viscositybonding agent while spinning the package directly formed on asemiconductor element, such as infrared ray detectors in which thesemiconductor element is used at a cryogenic temperature as low asnitrogen liquidifying temperature (77° K.).

In the die bonding, the color image sensor, the gold-silicon eutecticbonding method, or a solder which needs an elevated temperature cannotbe used since the color filter used is not heat-resistant. Therefore, abonding agent containing silver powders or a bonding agent of the typecured by ultraviolet rays is used. In the infrared ray detector, formedby the gold-silicon eutectic bonding method or a solder material are toorigid to accommodate the difference between the thermal expansions ofthe semiconductor element and the container vessel or the package. Thesebonds cause destruction of the semiconductor element at the bondedportion. Therefore, a low-temperature bonding agent which is relativelyflexible and strong at low temperatures is used in the infrared raydetector.

The low-temperature bonding agents generally used are an epoxy or aurethane bonding agent curable at room temperature. Such bonding agentsinclude Hisol (trade name) and Crest (trade name). These low-temperaturebonding agents when cured have a sufficient flexibility or softness aswell as a sufficient adhesion strength at cryogenic temperatures.However, since these bonding agents before curing have a high viscosity,of from 2,500 cps to 100,000 cps, it is very difficult to apply such thebonding agent to the surface to be bonded in a uniform thickness.

FIG. 1 illustrates, in cross section, a conventional semiconductordevice 1 of the type to which the present invention is applicable. Thesemiconductor device 1 comprises a semiconductor chip 2, a supportstructure which is illustrated as a package 3 for containing therein andsupporting the semiconductor element 2, and a bonding layer 4 of abonding agent disposed between the semiconductor element 2 and thepackage 2 for bonding the semiconductor element 2 on the package 3. Thebonding layer 4 is applied on the package 3 with a manual applicator(not shown) such as a putty knife or a brush to form a bonding surface 5bonded to the package 3 and a bonding surface 6 to which thesemiconductor element 2 is to be bonded. The bonding agent of thebonding layer 4 has a relatively high viscosity, so that the bondingsurface 6 of the layer 4, before the semiconductor element 2 is placedon it, is an irregular free surface. This irregular free surface hasprojections 7 and cavities 8. Then, the semiconductor element 2 ispressed onto the bonding surface 6 of the bonding layer 4, and thebonding agent of the bonding layer 4 is cured.

The semiconductor device 1 thus manufactured has a number of cavities 8or voids at the interface between the semiconductor element 2 and thebonding layer 4 because the relatively hard, highly viscous bondingagent of the bonding layer 4 is manually applied. Therefore, the bondingsurface 6 of the bonding layer 4 only partially contacts and adhereswith the semiconductor element 2, providing only a limited contact areabetween the die bonding region of the semiconductor element 2 and thepackage 3.

The voids formed by the cavities 8 between the semiconductor element 2and the bonding layer 4 lower the thermal conduction between thesemiconductor element 2 and the package 3. Also, during the thermalcycling of the semiconductor device 1 while in use, the moisture trappedwithin the voids 8 is repeatedly evaporated and condensed and the airpressure within the voids 8 changes. These factors significantly degradethe reliability of the semiconductor device.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide asemiconductor device in which the bonding surface of the bonding layeris smooth and has substantially no voids.

Another object of the present invention is to provide a semiconductordevice in which the voids in the bonding layer of the bonding agent areeliminated.

Still another object of the present invention is to provide asemiconductor device in which the semiconductor element is firmly andreliably bonded to the support structure.

A further object of the present invention is to provide a semiconductordevice in which a superior heat conduction can be established betweenthe semiconductor element and the support structure.

Another object of the present invention is to provide a process forbonding a semiconductor element to a support structure by a bondingagent with a high reliability.

A further object of the present invention is to provide a process forbonding a semiconductor element to a support structure by a bondingagent with substantially no voids.

With the above objects in view, the semiconductor device of the presentinvention comprises a semiconductor element, a support member forsupporting the semiconductor element and a bonding layer for bonding thesemiconductor element to the support member. The bonding layer comprisesa hard layer of a high-viscosity bonding agent which has a highviscosity before curing and which has an irregular free surface definingprojections and cavities, and a soft layer of a low-viscosity bondingagent which has a low viscosity before curing and is disposed within thecavities in the irregular free surface of the heard layer to fill them.The hard layer defines a portion of the bonding surfaces and the softlayer defines the remaining portion of the bonding surface to provide acontinuous hybrid bonding surface for the bonding layer.

According to the process for bonding a semiconductor element, ahigh-viscosity bonding agent which has a high viscosity before curing isapplied to a support member to provide a hard layer having an irregularfree surface defining projections and cavities therein. A low-viscositybonding agent which has a low viscosity before curing is applied to theirregular surface of the hard layer to fill the cavities with thelow-viscosity bonding agent to provide a soft layer. Thus, a continuoushybrid bonding surface which is partially defined by the projections ofthe hard layer and partially defined by the soft layer filled in thecavities of the hard layer is formed. Finally, a semiconductor elementis pressed onto the hybrid continuous bonding surface and the bondingagents are cured.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more readily apparent from thefollowing detailed description of the preferred embodiment of thepresent invention shown by way of example in the accompanying drawings,in which

FIG. 1 is a sectional view showing a conventional semiconductor devicein which the semiconductor element is bonded to the support member;

FIG. 2 is a sectional view showing a step of the semiconductor elementbonding process of the present invention in which a hard layer havingthe cavities in its surface is formed on the support member;

FIG. 3 is a sectional view showing a step of the process of the presentinvention in which a soft layer of a soft bonding agent is filled withinthe cavities of the hard layer; and

FIG. 4 is a sectional view showing a step of the process of the presentinvention in which semiconductor element is bonded to provide thesemiconductor device of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIGS. 2 to 4 illustrate, in cross section, various steps of thesemiconductor element bonding process of the present invention, FIG. 4showing a semiconductor device 11 in which the bonding process iscarried out according to the present invention. The semiconductor device11 shown in FIG. 4 comprises a semiconductor element 2, a support mmberwhich is illustrated as being a package 3 for containing therein andsupporting the semiconductor element 2, and a bonding layer 14 of abonding agent disposed between the semiconductor element 2 and thepackage 3 for bonding the semiconductor element 2 on the package 3. Thebonding layer 14 is bonded to the package 3 at the upper bonding surface14 and to the semiconductor element 2 at the lower bonding surface 16.The bonding layer 14 comprises a hard layer 12 and a soft layer 13disposed on the hard layer 12. It is seen that the hard layer 12 definesa portion of the upper bonding surface 16 and the soft layer defines theremaining portion of the upper bonding surface 16 to define together acontinuous hybrid bonding surface 17.

In FIG. 2, the hard layer 12 is applied on the chemically ormechanically cleaned surface on the package 3 with a manual applicator(not shown) such as a putty knife or a brush to form the bonding surface15 bonded to the package 3 and the bonding surface 16. The bonding agentof the hard layer 12 is a high-viscosity bonding agent which has arelatively high viscosity before curing, so that the bonding surface 16of the hard layer 12 before the semiconductor element 2 is placed on itis an irregular free surface 20 and has projections 18 and cavities 19.The high-viscosity bonding agent which may be used for the hard layer 12includes a low-temperature bonding agent or an epoxy resin bondingagent. The viscosity of such the bonding agent before curing maypreferably be from 2,500 cps. to 100,000 cps.

Then, as illustrated in FIG. 3, a low-viscosity bonding agent which hasa relatively low viscosity before curing is applied on the irregularfree surface 20 of the hard layer 12 to fill the cavities 19 with thelow-viscosity bonding agent to provide a soft layer 13. The soft layer13 may be applied by applying the low-viscosity bonding agent whilespinning the package for the purpose of removing excess low-viscositybonding agent from the irregular surface 20 of the hard layer 12. Thus,it is seen that a continuous hybrid bonding surface 17 which ispartially defined by the projections 18 of the hard layer 12 andpartially defined by the soft layer 13 filled in the cavities 19 of thehard layer 12 is provided on the bonding layer 14. The bonding agentwhich may be used for the soft layer 13 includes an urethane resinbonding agent. The viscosity of the bonding agent before curing may befrom 20 cps. to 500 cps.

In FIG. 4, a semiconductor element 2 is pressed onto the hybridcontinuous bonding surface 17 defined by the hard layer 12 and the softlayer 13 to firmly adhere the semiconductor element 2 to the package 3.The bonding surface of the semiconductor element 2 may be cleaned by asuitable chemical cleaning process before bonding.

With the above described semiconductor device, since a sufficiently highbonding strength can be obtained by the hard layer 12 of thehigh-viscosity bonding agent between the semiconductor element 2 and thepackage 3, the soft layer 13 of the low-viscosity bonding agent is notrequired to have a strong adhesion and is sufficient as a fillermaterial for the cavities 19. Therefore, the low-viscosity bonding agentcan be selected from a variety of bonding agents including a bondingagent having a viscosity of as low as from 20 cps. to 500 cps.

Also, since the low-viscosity bonding agent exhibits flexibility even ata very low temperature, the internal stresses which would otherwise begenerated in the semiconductor element 2 due to the difference inthermal expansion between the semiconductor element 2 and the package 3are absorbed, thereby virtually preventing the destruction of thesemiconductor element 2 by internal stresses.

Further, since the cavities 19 in the hard layer 12 are filled with thesoft layer 13 to define the hybrid continuous bonding surface 17,substantially no voids such as the voids 8 shown in FIG. 1 are formedbetween the semiconductor element 2 and the bonding layer 14. Therefore,the area of the effective bonding surface is large as compared to theconventional design, and the thermal conduction between thesemiconductor element 2 and the package 3 is significantly improved.

Further, since substantially no voids such as shown in FIG. 1 areformed, the hard and soft layers 12 and 13 are not undesirably affectedby the moisture trapped within the voids. Also, undesirable effects ofthe expansion and compression of the trapped air within the voids can besignificantly reduced, improving the reliability of the semiconductordevice.

What is claimed is:
 1. A process for bonding a semiconductor element toa package comprising:applying a relatively high-viscosity bonding agentto said package to provide a relatively hard layer having an irregularfree surface defining projections and cavities therein, the projectionsof said hard layer defining a portion of a bonding surface; applying arelatively low-viscosity bonding agent to said irregular free surface ofsaid hard layer to fill the cavities with said low-viscosity bondingagent to provide a relatively soft layer portion of said bondingsurface, said bonding surface being continuous and partially defined bysaid projections of said hard layer and partially defined by said softlayer filled in said cavities of said hard layer; and pressing asemiconductor element onto said continuous bonding surface defined bysaid hard layer and said soft layer.
 2. A process for bonding asemiconductor element as claimed in claim 1, wherein said high-viscositybonding agent is an epoxy resin and said low-viscosity bonding agent isan urethane resin.
 3. A process for bonding a semiconductor element asclaimed in claim 1 wherein said high-viscosity bonding agent has aviscosity of from 2,500 cps to 100,00 cps and said low-viscosity bondingagent has a viscosity of from 20 cps to 500 cps.
 4. A process forbonding a semiconductor element as claimed in claim 1 including applyinghigh-viscosity bonding agent with a manual applicator.
 5. A process forbonding a semiconductor element as claimed in claim 1 includingapplyings said low-viscosity bonding agent while spinning said package.